Mid-turbine frame spoke cooling system and method

ABSTRACT

A mid-turbine frame module comprises an outer structural ring, an inner structural ring and a plurality of circumferentially spaced-apart spokes structurally interconnecting the inner structural ring to the outer structural ring. At least one of the tubular spokes accommodates a service line. The remaining spokes with no service line have an internal architecture which mimics an air cooling scheme of the at least one spoke housing a service line in order to provide temperature uniformity across all spokes.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority on US Provisional Patent application No. 62/196,380 filed on Jul. 24, 2015, U.S. Provisional Patent Application No. 62/196,500 filed on Jul. 24, 2015 and US Provisional Patent Application No. 62/196,368 filed on Jul. 24, 2015, the entire content of the above applications is herein incorporated by reference.

TECHNICAL FIELD

The application relates generally to gas turbine engines and, more particularly, to a cooling arrangement for cooling the structural spokes of a mid-turbine frame module.

BACKGROUND OF THE ART

It is known to use structural spokes to transfer loads from a bearing casing to an outer structural ring of a gas turbine engine. For instance, such spokes may be found in mid-turbine frame modules. Each spoke typically extends radially from the outer ring through a strut in the gaspath to an inner ring supporting the bearing casing. During engine operation, the spokes all around the module must be maintained at substantially the same temperature in order to prevent the bearing from becoming off-centered as a result of differential thermal growth between the spokes.

SUMMARY

In one aspect, there is provided a mid-turbine frame module comprising an outer structural ring, an inner structural ring supporting a bearing, a plurality of circumferentially spaced-apart tubular spokes structurally interconnecting the inner structural ring to the outer structural ring, at least one of said tubular spokes accommodating a bearing service line, the remaining tubular spokes with no bearing service line having an internal architecture which mimics an air cooling scheme of the at least one spoke to provide temperature uniformity across all the spokes.

In accordance with another aspect, there is provided a tubular insert inside the tubular spokes, which house no service line, an annular gap being defined between said spokes and the insert.

In accordance with another aspect, flow calibration holes are provided to calibrate the cooling air through the annular gap.

In accordance with a further aspect, there is provided a mid-turbine frame for a gas turbine engine, the mid-turbine frame comprising: an outer structural ring, an inner structural ring, an annular gas path between the inner and outer structural ring, a plurality of circumferentially spaced-apart hollow struts extending radially across the gas path, a plurality of circumferentially spaced-apart tubular spokes respectively extending internally through the hollow struts, the tubular spokes structurally connected to the inner structural ring and to the outer structural ring, at least one of the tubular spokes housing a service line, a remainder of the tubular spokes having a sleeve extending therethrough, an internal coolant flow passage defined centrally through the sleeve and an annular coolant flow passage defined between the sleeve and the tubular spoke, the internal coolant flow passage and the annular coolant flow passage connected in serial flow communication at respective adjacent ends thereof and with a source of coolant liquid to provide a coolant reverse flow path from a radially inward direction to a radially outward direction.

In accordance with another further aspect, there is provided a spoke cooling arrangement for a gas turbine engine mid-turbine frame module comprising a plurality of circumferentially spaced-apart tubular spokes structurally interconnecting an inner structural ring to an outer structural ring, at least one of the tubular spokes housing a service line, the spoke cooling arrangement comprising: a main coolant flow passage extending through each of the spokes having no service line, and a reverse flow passage serially interconnected to the main coolant flow passage for recirculating at least a portion of the coolant back into the associated spoke in a direction opposite to that of the main coolant flow passage.

In accordance with a still further general aspect, there is provided a method of cooling structural spokes of a gas turbine engine mid-turbine frame module, wherein at least one of the structural spokes houses a service line; for each of the structural spokes housing no service line, the method comprising: directing a coolant flow radially inwardly through a main flow passage defined axially through the structural spokes, and redirecting at least a portion of the coolant flow received from the main flow passage radially outwardly into a reverse flow passage extending axially through each of the structural spokes with no service line.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a schematic cross-section view of a gas turbine engine;

FIG. 2 is an isometric view of a mid-turbine frame module mounted in an engine outer case;

FIG. 3 is an isometric view of the mid-turbine frame shown without the engine outer case;

FIG. 4 is an enlarged view of a portion of the mid-turbine frame illustrating an air intake arrangement for uniformly distributing cooling air all around the module and avoid the formation of a local cold spot in the module;

FIG. 5 is a cross-section view of the air intake arrangement shown in FIG. 4;

FIG. 6a is a cross-section view of a portion of the mid-turbine frame module illustrating a cooling flow scheme through one of the spokes;

FIG. 6b is an enlarged view of a radially inner end portion of the spoke cooling flow scheme shown in FIG. 6 a;

FIG. 6c is an enlarged view of a radially outer end portion of the spoke cooling flow scheme shown in FIG. 6 a;

FIG. 7a is an end view of the mid-turbine frame module illustrating a first cooling circuit for structurally dedicated spokes, which do not accommodate any service lines, and a second cooling circuit for the top and bottom spokes, which integrate bearing housing service lines, the two circuits being separated to avoid air contamination;

FIG. 7b is an enlarged cross-section view of a radially inner outlet end portion of the first cooling circuit; and

FIG. 7c is an enlarged cross-section view of a radially outer outlet end portion of the second cooling circuit.

DETAILED DESCRIPTION

FIG. 1 illustrates a turbofan gas turbine engine 10 of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan 12 through which ambient air is propelled, a multistage compressor 14 for pressurizing the air, a combustor 16 in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section 18 for extracting energy from the combustion gases.

FIGS. 2 and 3 show a portion of the turbine section 18. More particularly, FIG. 2 illustrates a mid-turbine frame module 20 housed within an engine outer case 21. As shown in FIG. 3, the mid-turbine frame module 20 comprises an inner structural ring 22 adapted to receive and support a bearing casing 23, which is, in turn, adapted to support the main shafts of the engine 10. The bearing casing 23 may be detachably mounted to the inner ring 22 by means of bolts or the like.

The inner bearing support ring 22 is structurally supported by an outer structural ring 24 by means of a plurality of circumferentially distributed tubular spokes 26 (6 in the illustrated embodiment). In addition of transferring the loads from the inner ring 22 to the outer ring 24, the spokes 26 centralize the inner ring 22 and, thus, the bearing casing 23 relative to the outer ring 24. The term “tubular spoke” is herein intended to generally refer to a hollow spoke structure and is not limited to any specific cross-sectional shape.

Each spoke 26 may extend radially through a hollow strut 29 a, b (FIG. 6a ) of a non-structural integrated strut-vane (ISV) casing 28 “floatingly” mounted between the inner and outer structural rings 22 and 24 for guiding the combustion gases between two axially adjacent turbine stages. The ISV casing 28 has a radially outer and a radially inner gaspath walls 28 a, 28 b (FIGS. 5 and 6 a) defining therebetween a portion of the gaspath of the turbine section 18. According to the illustrated embodiment, the ISV casing 28 does not play a structural role. That is loads from the bearing casing 23 are not transmitted to the outer casing 24 via the ISV casing 28. The loads are rather transmitted through the spokes 26, which are shielded from the hot combustion gases by the hollow struts 29 of the ISV casing 28. In such an arrangement, the spokes can be referred to as cold spokes.

During engine operation, all the spokes 26 need to be kept at substantially the same temperature in order to prevent the bearing casing 23 from becoming off-centered. Indeed, if the spokes 26 have different thermal growths, the concentricity of the inner ring 22 relative to the outer ring 24 may be lost and consequently the bearing centralization compromised. Accordingly, there is a need for a way to uniformly distribute coolant to the spokes 26 all around the module 20 so that the temperature of all the spokes 26 is substantially the same. Moreover, when introducing coolant (e.g. compressor bleed air) in module 20, the coolant should be directed such as to avoid creating local cold spots on the outer ring 24, which could also affect the bearing centralization.

According to one embodiment, a single external pipe (not shown) may be used to direct coolant, such as bleed air from the compressor of the engine 10, to the mid-turbine frame module 20. As shown in FIG. 2, a port 30 is provided on the engine outer case 21 for receiving cooling air from the external pipe. Cooling air from the engine outer case intake port 30 is then directed into an intake duct 32 mounted to the outer structural ring 24. According to the embodiment illustrated in FIG. 4, the intake duct 32 may be provided in the form of a generally T-shaped duct having an inlet branch 32 a extending radially through a hole 34 defined in the outer ring 24 and a pair of outlet branches 32 b extending laterally from opposed sides of the inlet branch 32 a on a radially inner side of the outer ring 24. The outlet branches 32 b generally extend in circumferentially opposite directions and have respective outlet ends connected to outlet ports 36 provided on the outer ring 24 on opposed sides of the hole 34. The intake duct 32 may be made in sheet metal, casting or any other suitable materials.

As shown in FIG. 5, the outlet branches 32 b of the air intake duct discharge the cooling air in circumferentially opposed directions into an annular cavity 40 defined between the engine outer case 21 and the outer ring 24. The annular cavity 40 forms an air plenum all around the module. As shown in FIG. 3, the air plenum is in flow communication with the spokes and the hollow struts in which the spokes 26 are positioned. By building an air pressure in the annular air plenum, cooling air may be uniformly distributed to the spokes 26 all around the cavity 40. It provides for an internal core passage architecture that distributes the cooling air in a circumferential manner to avoid unequal metal temperature in the mid-turbine frame module outer ring structure. Also, it can be appreciated that the air intake duct 32 prevents the incoming cooling air to be locally discharged directly against the outer ring 24, thereby avoiding the creation of a local cold spot thereon adjacent one of the spokes 26. The air intake duct 32 rather splits the incoming flow of cooling air and redirects it with a radially outward and a circumferential component into the annular cavity 40 between the outer ring 24 and the engine outer case 21. The air impacts upon the engine outer case 21 and, thus, not on the outer ring 24, which is used to centralize the inner bearing casing 23 with the spokes 26. This contributes preserving the bearing centralization.

Also the above embodiment eliminates the use of multiple air cooling feed pipes, which may have a non-negligible impact on the overall weight of the engine. It also allows the introduction of cooling air in a restricted area. The air duct internal intake can also be easily replaced.

According to an embodiment, six spokes are used to support and centralize the bearing casing 23. Two of the spokes 26 (one at the bottom and one at the top of the module) are also used to accommodate bearing housing service lines 50, such as oil tubes. FIG. 6a illustrates an example of a first hollow airfoil strut 29 a containing a combined structural spoke 26 a and bearing housing service line 50 and a second hollow airfoil strut 29 b containing a structurally dedicated spoke 26 b (spoke with no oil service lines). The two structural spokes 26 a with their internal bearing service lines 50 and the four structurally dedicated spokes 26 b must be kept at substantially the same temperature to ensure rotor centralization. This may be achieved by providing in each of the 4 structurally dedicated spokes with an internal architecture that mimics the air circulation through the 2 spokes accommodating the bearing service lines 50.

Referring concurrently to FIGS. 6a to 6c , it can be appreciated that a sleeve or tubular insert 52 may be provided in each of the 4 structurally dedicated spokes 26 b to form an internal annular gap or annular reverse flow passage 54, which generally corresponds to the one between the combined spoke 26 a and bearing housing service line 50 and associated surrounding strut 29 a. Referring concurrently to FIGS. 6a to 6c and 7a , it can be appreciated that a first cooling circuit is formed between the annular cavity 40 and the 4 structurally dedicated spokes 26 b. The cooling air flows from the annular cavity 40 radially inwardly through the internal main coolant flow passage defined by the tubular insert 52 mounted inside each of the structurally dedicated spokes 26 b. As shown in FIG. 7b , the air discharged from the insert 52 of each spoke 26 b is received in a chamber 80 defined between the inner ring 22 and the radially inner end of each spoke 26 b. A first portion of this air is discharged through holes 82 in the inner ring 22 and then directed to purge the upstream disc cavity 93 of an adjacent turbine rotor 95. As best shown in FIG. 6b , the remaining portion of the cooling air discharged from each insert 52 is recirculated back through the spokes 26 b in the annular reverse flow passage 54. Flows calibrating holes or other suitable flow calibration devices 56 are provided at the radially inner end of each spoke 26 b to calibrate the flow of cooling air passing through each of the annular gaps 54. The holes 56 are calibrated so that the portion of the cooling air flowing radially outwardly through the annular gap 54 maintains the spokes 26 b substantially at the same temperature as the top and bottom spokes 26 a housing the internal bearing service lines 50. As shown in FIG. 6c , outlet holes 58 are defined in the radially outer end portion of the spokes 26 b to discharge the cooling air between the ISV casing 28 and the outer ring 24. This flow path mimics the cooling flow path around the top and bottom spokes 26 a (FIG. 7a ) used for the oil tubes/bearing service lines 50. This configuration ensures that all the structural spokes 26 with and without bearing housing service lines are kept at the same temperature, thereby ensuring bearing housing centralization throughout the engine operating envelope. In the prior art, separate struts had to be used for the structural spokes and the bearing service lines. With the new proposed arrangement, a service line and a spoke can be positioned in a same hollow strut. This reduces the number of large, hollow struts in the gaspath. It allows the cold spoke design mid-turbine frame to be used in physically smaller engines. The uniformity of the cooling flow between the different types of spokes ensures bearing housing concentricity while allowing various hardware combinations to transverse the ISV gaspath combinations.

Referring to FIGS. 7a and 7c , it can be appreciated that the cooling system comprises a second cooling circuit which is separate from the first cooling circuit described above for the 4 structurally dedicated spokes 26 b. The second cooling circuit provides cooling to the top and bottom spokes 26 a housing the service lines 50. As can be appreciated from FIG. 7a , the annular gap between the bottom spoke 26 a and the service line 50 extending therethrough is connected in fluid flow communication with the annular cavity or air plenum 40. The air is discharged from the bottom spoke 26 a into a sealed annular chamber or cavity 90 defined between the inner ring 22, the bearing casing 23 and a rear cover 92 (FIG. 7b ) bolted to the inner ring 22. The cooling air travels circumferentially through the annular cavity 90 from the bottom spoke 26 a to the top spoke 26 a. As shown in FIG. 7a , the cooling air exits the annular cavity 90 via the annular gap defined between the top spoke 26 a and the service line 50 extending therethrough. As shown in FIG. 7c , the air is discharged at a radially outer end of the service line 50 through outlet holes 94. The person skilled in the art will appreciate that the top and bottom spokes 26 a are used to feed/purge air and oil of a scupper line in the case of oil failure. The air in the first circuit through the 4 structurally dedicated spokes 26 b will not be contaminated by the air flowing through the top and bottom spoke housing the service lines 50 in the event of oil leakage.

The use of the 4 structurally dedicated spokes 26 b to feed secondary cooling air from the cavity 40 to the cavity disc of the upstream rotor also contributes to reduce the number of pipes and tubes. Indeed, the spokes are used as air feed tubes to direct cooling air to adjacent turbine components, thereby reducing the number of parts to be installed on the engine.

The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. Any modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims. 

The invention claimed is:
 1. A casing assembly for a gas turbine engine, the casing assembly comprising: an engine outer case extending circumferentially about a central axis; and a mid-turbine frame housed within the engine outer case, the mid-turbine frame including an outer structural ring disposed concentric with the central axis, an annular air plenum extending all around the outer structural ring, the annular air plenum radially bounded by a radially inner surface of the engine outer case and a radially outer surface of the outer structural ring, an inner structural ring disposed concentric with the central axis, an annular gas path between the inner and outer structural ring, a plurality of circumferentially spaced-apart hollow struts extending radially across the gas path, a plurality of circumferentially spaced-apart tubular spokes respectively extending internally through the hollow struts, the tubular spokes structurally connected to the inner structural ring and to the outer structural ring independently of the engine outer case, the tubular spokes defining a load path for transferring loads from the inner structural ring to the outer structural ring, at least one of the tubular spokes housing a service line, a remainder of the tubular spokes having a sleeve extending therethrough, an internal coolant flow passage defined through the sleeve and an annular coolant flow passage defined between the sleeve and the tubular spoke, the internal coolant flow passage and the annular coolant flow passage connected in serial flow communication at respective adjacent ends thereof and with a source of coolant to provide a coolant reverse flow path from a radially inward direction to a radially outward direction, and wherein the remainder of the tubular spokes are connected at respective radially outer ends thereof in fluid flow communication with the annular air plenum, the annular air plenum configured to convey cooling air to the remainder of the tubular spokes all around the outer structural ring.
 2. The casing assembly defined in claim 1, further comprising a flow calibrating device between the internal coolant flow passage and the annular coolant flow passage.
 3. The casing assembly defined in claim 2, wherein the flow calibrating device is provided in the form of flow calibrating holes defined at a radially inner end of the remainder of tubular spokes.
 4. The casing assembly defined in claim 1, wherein a chamber is defined between the inner ring and a radially inner end of the remainder of the tubular spokes, the chamber being in fluid flow communication with both the internal coolant flow passage and the annular coolant flow passage, and wherein purge holes are defined in the chamber, the purge holes being in flow communication with an upstream disc cavity of an adjacent turbine rotor.
 5. The casing assembly defined in claim 1, further comprising an intake duct having an inlet branch and a pair of outlet branches extending laterally from opposed sides of the inlet branch, the outlet branches extending generally in circumferentially opposite directions and having respective outlet ends in flow communication with the annular air plenum, and wherein the outlet ends are oriented to discharge cooling air with a radially outward component into the annular air plenum.
 6. The casing assembly defined in claim 5, wherein the intake duct is mounted to a radially inner side of the outer structural ring, and wherein the outlet ends of the outlet branches are connected to respective outlet ports defined through the outer structural ring on opposed sides of a central inlet port defined in the outer structural ring and to which the inlet branch of the intake duct is mounted.
 7. The casing assembly defined in claim 2, wherein the flow calibrating device is calibrated so that, in use, the coolant flowing radially outwardly through the annular coolant flow passage maintains the reminder of the tubular spokes at a temperature corresponding to a temperature of the at least one spoke housing the service line, thereby allowing for temperature uniformity across all the structural spokes.
 8. The casing assembly defined in claim 1, wherein outlet holes are defined in a radially outer end portion of the remainder of the tubular spokes to discharge coolant from the annular coolant flow passage.
 9. The casing assembly defined in claim 1, wherein the internal coolant flow passage has an inlet and an outlet respectively provided at a radially outer end and a radially inner end of the sleeve, and wherein the annular coolant flow passage has an inlet and an outlet respectively provided at a radially inner end and a radially outer end of the associated spoke.
 10. The casing assembly defined in claim 1, wherein the inner structural ring supports a bearing housing, and wherein the service line comprises a bearing housing service line.
 11. A spoke cooling arrangement for a gas turbine engine mid-turbine frame module housed in an engine outer case, the gas turbine engine mid-frame module comprising a plurality of circumferentially spaced-apart tubular spokes structurally directly interconnecting an inner structural ring to an outer structural ring disposed concentric to a central axis the gas turbine engine mid-turbine frame, the tubular spokes defining a load path through which loads on the inner structural ring are transferred to the outer structural ring, the outer structural ring and the engine outer case defining an annular air plenum radially therebetween, the annular air plenum extending circumferentially all around the outer structural ring, at least one of the tubular spokes housing a service line, the spoke cooling arrangement comprising: a main coolant flow passage extending through each of the spokes having no service line, and a reverse flow passage serially interconnected to the main coolant flow passage for recirculating at least a portion of a coolant flowing from the main coolant flow passage back into the associated spoke in a direction opposite to that of the main coolant flow passage, the main coolant flow passage in flow communication with the annular air plenum disposed radially outwardly of the outer structural ring, the annular air plenum distributing cooling air in a circumferential manner between the tubular spokes.
 12. The spoke cooling arrangement defined in claim 11, wherein the main coolant flow passage has an inlet and an outlet respectively provided at a radially outer end and a radially inner end of the associated tubular spoke, and wherein the reverse flow passage has an inlet and an outlet respectively provided at the radially inner end and the radially outer end of the associated tubular spoke.
 13. The spoke cooling arrangement defined in claim 11, wherein, in use, the coolant travels through the main coolant flow passage in a radially inward direction, the reverse flow passage receiving the coolant from the main coolant flow passage for recirculating through the associated spoke in a radially outward direction.
 14. The spoke cooling arrangement defined in claim 11, wherein a tubular insert is provided within each of the tubular spokes with no service line, the main coolant flow passage extending through the tubular insert, and wherein the reverse flow passage is defined by an annulus between the tubular insert and the associated tubular spoke.
 15. The spoke cooling arrangement defined in claim 11, further comprising a flow calibrating device between the main coolant flow passage and the reverse flow passage.
 16. The spoke cooling arrangement defined in claim 15, wherein the flow calibrating device is provided in the form of flow calibrating holes defined at a radially inner end of each of the tubular spokes with no service line.
 17. The spoke cooling arrangement defined in claim 11, wherein each of the tubular spokes is shielded from an annular gas path between the inner and outer structural rings by a hollow strut.
 18. A method of cooling structural spokes of a gas turbine engine mid-turbine frame module having an outer structural ring and an inner structural ring with the structural spokes extending therebetween, the outer structural ring and the inner structural ring disposed concentric with a central axis of the gas turbine engine mid-turbine frame module, the structural spokes transferring loads from the inner structural ring directly to the outer structural ring, the method comprising: directing a coolant flow in an annular air plenum encircling the outer structural ring; building air pressure in the annular air plenum outside of the outer structural ring; distributing the coolant flow from the annular air plenum to the structural spokes all around the annular air plenum; including directing the coolant flow radially inwardly through a main flow passage defined axially through the structural spokes, and redirecting at least a portion of the coolant flow received from the main flow passage radially outwardly into a reverse flow passage extending axially through the structural spokes.
 19. The method defined in claim 18, wherein a tubular insert is provided in each of the structural spokes housing no service line, and wherein directing the coolant flow radially inwardly through the main flow passage comprises directing the coolant radially inwardly through the tubular insert, and wherein directing the coolant flow radially outwardly into the reverse flow passage comprises directing the coolant flow received from the tubular insert into an annulus between the tubular insert and the associated structural spoke.
 20. The casing assembly defined in claim 1, wherein the tubular spokes are directly attached to the outer structural ring. 